Unit value is defined as the value of 1 metric ton (t) apparent consumption of arsenic content. Excluding the year 1908, data are estimated using the market price in U.S. dollars per ton of arsenic trioxide. Data for 1908 is the average of the 1908 market price range for arsenic trioxide. The market price of arsenic trioxide was converted to a value for the contained arsenic by dividing the arsenic trioxide price by the percentage of arsenic contained in arsenic trioxide (75.7 percent). Data for the years 1900–96 are from the MR and the MYB. Data for the years 1997–2004 are from the MCS.

Unit value is defined as the value of 1 metric ton (t) of asbestos apparent consumption. Data for the years 1900–87 and 1991–94 are calculated based on the asbestos sold or used value data from the MR and MYB. Data for the years 1988–90 and 1995–2004 are calculated based on the asbestos import value data from the MYB. The major change in value in the years 2003–04 from previous years reflects a change to higher valued material.

Unit value is the value in dollars of 1 metric ton (t) of natural and synthetic diamond apparent consumption. Unit value data for the years 1900–2004 were estimated as being equal to the total value of imports divided by the total import quantity. Unit values rose steeply from 1926 to 1927–29 due to the added demand for glaziers’ and engravers’, unset, and miners’ diamond and decreased in 1930 when demand subsided. Unit value decreased, especially during the late 1980s to 2004, due to an increasing amount of synthetic diamonds being produced.

Unit value is the value in actual U.S. dollars of 1 metric ton (t) of refined copper apparent consumption. Unit value is estimated from the “Annual Average U.S. Producer Copper Price” as reported in the MP98 and the 2004 MYB.

Unit value is the value in actual U.S. dollars of 1 metric ton (t) of gold apparent consumption. Unit values were estimated using the Englehard market prices for refined gold as reported in the MP98 and the 2004 MYB.

Unit value is defined as the value in dollars of 1 metric ton (t) of pig iron apparent consumption. Unit value for pig iron is estimated from the reported average annual value of shipments from the MYB and the MR. Data were not available for the years 1989–2004.

Unit value is defined as the value of 1 metric ton (t) of silver apparent consumption. Unit value data for the years 1900–98 were recorded from the MP98. Unit value data for the years 1999–2004 were recorded from the MYB. Unit value data for the years 1900–2004 represent the average value per metric ton of silver of a minimum purity of 99.9 percent.